As a general rule, discards are done when the target resource is not available.
For a transmit, the lacking resource is the client. For example, if sleep mode is enabled, the AP will store some outbound traffic until the sleeping client wakes up and requests it. If the client drops (times out, turned off, walked off, or goes brain dead), that traffic would be discarded.
Compare the configuration for that AP to other nearby and see if there's some parameter set differently. Also take a critical look at the location of the AP and the expected client traffic; would you be getting a lot of pass-through traffic? or could the clients be switching between this AP and another near by?
What's your best guess as to why this might be happening?
What kind of AP and is it stand-alone or centrally controlled?
All the indivdual levels have the same number of equally space AP but not much client are using it at that level compare to the AP in other levels where there are lof of clients and with lower transmit errors.
Therefore, it is less likely to switch between the nearby AP. The AP are standalone and all the AP have the same configuration. My guess is the DotRadio is intermittent faulty.
By the way, how to check the number of pass-through traffic?